This application is based on Japanese Patent Application No. 2000-83247, filed on Mar. 21, 2000, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a real-image variable-magnification viewfinder, and more particularly to a real-image variable-magnification viewfinder that offers a high zoom ratio and is suitable for use in a digital camera.
2. Description of Prior Art
As image-sensing devices such as CCDs (charge-coupled devices) for use in digital cameras become smaller and smaller, there has been an increasingly greater demand for compact high-zoom-ratio real-image viewfinders. However, most conventional real-image variable-magnification viewfinders offer zoom ratios of 3xc3x97 or lower, and those offering zoom ratios over 3xc3x97 are large. For example, the real-image variable-magnification viewfinder proposed in Japanese Patent Application Laid-Open No. H6-102453 has an objective optical system composed of, from the object side, a positive lens unit, a negative lens unit, a positive lens unit, and a fourth lens unit including at least one negative lens element. However, this viewfinder offers a zoom ratio of only about 2xc3x97 and lower. The real-image variable-magnification viewfinder proposed in U.S. Pat. No. 5,225,927 has an objective optical system composed of, from the object side, a positive lens unit, a negative lens unit, a positive lens unit, and a fourth lens unit that has a weak optical power. However, this viewfinder, too, offers a zoom ratio of up to about 2.5xc3x97 to 3.5xc3x97, and is, in addition, large.
The real-image variable-magnification viewfinder proposed in U.S. Pat. No. 5,694,244 has an objective optical system composed of, from the object side, a positive lens unit, a negative lens unit, and a positive lens unit. This viewfinder offers a zoom ratio over 3xc3x97, but is large. The real-image variable-magnification viewfinder proposed in U.S. Pat. No. 6,035,145 has an objective optical system composed of four lens units arranged in the following order, from the object side, a positive lens unit, a negative lens unit, a positive lens unit, and a negative lens unit. This viewfinder achieves zooming from the wide-angle end to the telephoto end by moving the second lens unit toward the image-plane side and moving the third lens unit monotonically toward the object side. This viewfinder offers a zoom ratio of up to about 4xc3x97, but is rather large.
As proposed in Japanese Patent Application Laid-Open No. H6-102453 and U.S. Pat. No. 5,225,927 mentioned above, in conventional real-image variable-magnification viewfinders having an objective optical system starting with a positive lens unit, a negative lens unit, and a positive lens unit, zoom solutions are commonly designed so that the second lens unit is almost solely responsible for zooming. Such zoom solutions, however, inevitably require a long movement distance in the second lens unit and a high power in the third lens unit. As a result, large aberration is caused by the third lens unit and large variations occur in the aberration caused by the second lens unit as zooming is performed. This makes miniaturization of such viewfinders impossible. On the other hand, giving the second and third lens units approximately equal shares of the overall zoom ratio as proposed in U.S. Pat. Nos. 5,694,244 and 6,035,145, mentioned above, helps miniaturize such viewfinders, but only to an insufficient degree.
An object of the present invention is to provide a real-image variable-magnification viewfinder that, despite having a high zoom ratio of over 3xc3x97, is compact and offers satisfactory optical performance.
To achieve the above object according to one aspect of the present invention, a real-image variable-magnification viewfinder is provided with: an objective optical system having a positive optical power for forming a real image in an optical path; an eyepiece optical system having a positive optical power for transmitting the real image formed by the objective optical system to a pupil of an observer; and an erecting optical system disposed in the optical path for inverting the real image formed by the objective optical system. The objective optical system is provided with three lens units arranged in the following order, from the object side; a first lens unit having a positive optical power; a second lens unit having a negative optical power; and a third lens unit having a positive optical power. In this viewfinder, as zooming is performed from the wide-angle end to the telephoto end, at least one of the second and third lens units are moved in such a way that the second and third lens units come closer to each other. In addition, the following conditional formulae are fulfilled:
xe2x88x920.75 less than m2W less than xe2x88x920.3
xe2x88x922 less than m2T less than 1.05
xe2x88x920.75 less than m3W less than xe2x88x920.3
xe2x88x922 less than m3T less than xe2x88x921.05
L2 greater than L3
where
m2W represents the lateral magnification of the second lens unit at the wide-angle end;
m2T represents the lateral magnification of the second lens unit at the telephoto end;
m3W represents the lateral magnification of the third lens unit at the wide-angle end;
m3T represents the lateral magnification of the third lens unit at the telephoto end;
L2 represents the movement distance of the second lens unit over the entire zoom range; and
L3 represents the movement distance of the third lens unit over the entire zoom range.
According to another aspect of the present invention, a real-image variable-magnification viewfinder is provided with an objective optical system having a positive optical power for forming a real image in an optical path; an eyepiece optical system having a positive optical power for transmitting the real image formed by the objective optical system to a pupil of an observer; and an erecting optical system disposed in the optical path for inverting the real image formed by the objective optical system. The objective optical system is provided with four lens units arranged in the following order, from the object side; a first lens unit having a positive optical power; a second lens unit having a negative optical power; a third lens unit having a positive optical power; and a fourth lens unit. In this viewfinder, as zooming is performed from the wide-angle end to the telephoto end, at least one of the second and third lens units are moved in such a way that the second and third lens units come closer to each other. In addition, the following conditional formulae are fulfilled:
xe2x88x920.75 less than m2W less than xe2x88x920.3
xe2x88x922 less than m2T less than xe2x88x921.05
xe2x88x920.75 less than m3W less than xe2x88x920.3
xe2x88x922 less than m3T less than xe2x88x921.05
L2 greater than L3
xe2x88x920.1 less than PW4 less than 0.04
where
m2W represents the lateral magnification of the second lens unit at the wide-angle end;
m2T represents the lateral magnification of the second lens unit at the telephoto end;
m3W represents the lateral magnification of the third lens unit at the wide-angle end;
m3T represents the lateral magnification of the third lens unit at the telephoto end;
L2 represents the movement distance of the second lens unit over the entire zoom range;
L3 represents the movement distance of the third lens unit over the entire zoom range; and
PW4 represents the optical power (mmxe2x88x921) of the fourth lens unit.
According to another aspect of the present invention, a real-image variable-magnification viewfinder is provided with: an objective optical system having a positive optical power for forming a real image in an optical path; an eyepiece optical system having a positive optical power for transmitting the real image formed by the objective optical system to a pupil of an observer; and an erecting optical system disposed in the optical path for inverting the real image formed by the objective optical system. The objective optical system is provided with four lens units arranged in the following order, from the object side: a first lens unit having a positive optical power; a second lens unit having a negative optical power; a third lens unit having a positive optical power; and a fourth lens unit having a negative optical power. In this viewfinder, as zooming is performed, the first, second, and third lens units are moved. In addition, the following conditional formulae are fulfilled:
xe2x88x920.95 less than m2W less than xe2x88x920.3
xe2x88x923 less than m2T less than xe2x88x921.05
xe2x88x920.95 less than m34W less than xe2x88x920.3
xe2x88x922.6 less than m34T less than xe2x88x921.05
where
m2W represents the lateral magnification of the second lens unit at the wide-angle end;
m2T represents the lateral magnification of the second lens unit at the telephoto end;
m34W represents the composite lateral magnification of the third and fourth lens units at the wide-angle end; and
m34T represents the composite lateral magnification of the third and fourth lens units at the telephoto end.
According to another aspect of the present invention, a real-image variable-magnification viewfinder is provided with: an objective optical system having a positive optical power for forming a real image in an optical path; an eyepiece optical system having a positive optical power for transmitting the real image formed by the objective optical system to a pupil of an observer; and an erecting optical system disposed in the optical path for inverting the real image formed by the objective optical system. The objective optical system is provided with at least three lens units arranged in the following order, from the object side: a first lens unit, and at least two succeeding lens units. In this viewfinder, the first lens unit is moved for adjustment of dioptric power, and zooming is achieved by moving at least two of the succeeding lens units in such a way that magnification is variable within a range extending to both sides of unity magnification.